Datasheet

ADS1110
SBAS276A − MARCH 2003 − REVISED NOVEMBER 2003
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15
SINGLE-ENDED INPUTS
Although the ADS1110 has a fully differential input, it can
easily measure single-ended signals. A simple
single-ended connection scheme is shown in Figure 8.
The ADS1110 is configured for single-ended
measurement by grounding either of its input pins, usually
V
IN−
, and applying the input signal to V
IN+
. The
single-ended signal can range from 0V to 2.048V. The
ADS1110 loses no linearity anywhere in its input range.
Negative voltages cannot be applied to this circuit
because the ADS1110 inputs can only accept positive
voltages.
Figure 8. Measuring Single-Ended Inputs
The ADS1110 input range is bipolar differential with
respect to the reference, i.e. 2.048V. The single-ended
circuit shown in Figure 8 covers only half the ADS1110
input scale because it does not produce differentially
negative inputs; therefore, one bit of resolution is lost.
LOW-SIDE CURRENT MONITOR
Figure 9 shows a circuit for a low-side shunt-type current
monitor. The circuit reads the voltage across a shunt
resistor, which is sized as small as possible while still
giving a readable output voltage. This voltage is amplified
by an OPA335 low-drift op amp and the result is read by the
ADS1110.
Figure 9. Low-Side Current Measurement
It is suggested that the ADS1110 be operated at a gain of
8. The gain of the OPA335 can then be set lower. For a gain
of 8, the op amp should be set up to give a maximum output
voltage of no greater than 0.256V. If the shunt resistor is
sized to provide a maximum voltage drop of 50mV at
full-scale current, the full-scale input to the ADS1110 is
0.2V.
ADVICE
The ADS1110 is fabricated in a small-geometry
low-voltage process. The analog inputs feature protection
diodes to the supply rails. However, the current-handling
ability of these diodes is limited, and the ADS1110 can be
permanently damaged by analog input voltages that
remain more than approximately 300mV beyond the rails
for extended periods. One way to protect against
overvoltage is to place current-limiting resistors on the
input lines. The ADS1110 analog inputs can withstand
momentary currents of as large as 10mA.
The previous paragraph does not apply to the I
2
C ports,
which can both be driven to 6V regardless of the supply.
If the ADS1110 is driven by an op amp with high-voltage
supplies, such as ±12V, protection should be provided,
even if the op amp is configured so that it does not output
out-of-range voltages. Many op amps seek to one of the
supply rails immediately when power is applied, usually
before the input has stabilized; this momentary spike can
damage the ADS1110. Sometimes this damage is
incremental and results in slow, long-term failure—which
can be disastrous for permanently installed,
low-maintenance systems.
If an op amp or other front-end circuitry is used with the
ADS1110, its performance characteristics must be taken
into account. A chain is only as strong as its weakest link.